Dyes may be used for the dyeing of several very different materials like e.g. natural and synthetic fibers, textiles, plastics and leather. Dyes may be used further as acid-base indicators, as biological stains, as filter dyes, antihalation dyes or dyes providing more sharp images in photographic applications, for information recording, printing inks, camouflage, as light-collecting elements, for the coloring of food, as an analytical reagent, as a transfer dye in dye-transfer photographic materials and in many other applications.
In photography dyes may be divided into spectrally and non-spectrally sensitising dyes, indicating whether the dyes are adsorbed or not at the surface of the light-sensitive silver halide crystals used in photographic elements.
Non-spectrally sensitising dyes e.g. are widely used in a photosensitive silver halide emulsion layer as screening dyes, in an undercoat adjacent to the photosensitive layer and/or in a backing layer on the side of the support opposite to the photosensitive layer(s) to absorb reflected and scattered light. Its function as antihalation dye or in an overcoat or interlayer to shield a particular photosensitive layer against undesired exposure being therefore referred to as filter or absorber dye is highly appreciated. The adjustment of the sensitivity of a photographic element as required in the production specifications is another application.
For example in order to improve image sharpness an absorber dye can be present in one or more filter layers between silver halide emulsion layers that are coated at opposite sides of a transparent film support of an X-ray recording material. Imagewise exposure of said recording material proceeds in a cassette between a pair of X-ray intensifying screens that each are held in contact with an adjacent silver halide emulsion layer. By said arrangement the imaging light that would cross the support and to some extent becomes scattered thereby, is considerably attenuated and cannot give rise to an unsharp image in an opposite silver halide emulsion layer.
Spectrally the dye absorption spectrum should approximately be equal to the sensitivity spectrum of the corresponding silver halide emulsion in the layer of which a sharp image has to be reproduced.
On one hand it is very important that filter dyes remain i.e. that they are non-migratory, in the layer wherein they have been incorporated especially when this layer is in direct contact with the silver halide emulsion layer in order to prevent a desensitising action on the silver halide. At the other hand the filter dyes may not stain the photographic material after image processing. Therefore preference is given to filter dyes that decolorise or can be removed from the photographic element in the processing cycle. This requirement is nowadays becoming more and more stringent as rapid processing times are of increasing interest.
As described in U.S. Pat. No. 3,560,214 dyes comprising a carboxyl and phenyl substituted pyrazoline nucleus linked through a methine group to a dialkylaminophenyl group can be removed relatively easily in alkaline aqueous processing liquids but lack sufficient fastness to diffusion in hydrophilic colloid layers.
Other filter dyes characterised by the presence of a 2-pyrazolin-5-one nucleus substituted with a carboxyphenyl group and including a methine group or chain linked to a dialkylamino group are described in U.S. Pat. No. 4,857,446.
Recently in EP-A's 0 586 748, 0 587 229, 0 587 230 and 0 656 401 the synthesis and the application of new dyes have been described, wherein the filter dyes have an ester, an amide, a nitrile function or a derivative therefrom as a substituent at the mono- or trimethine chain, or wherein said dyes show the presence of a pyrrolo2,3-c! pyrrazolone ring. When said new dyes are incorporated in the non-migratory state in hydrophilic colloid layers of photographic materials they can be rapidly removed after being quickly decolorised under the influence of sulphite ions in alkaline aqueous liquids used in the processing of said materials.
As is well-known the monomethine dyes have an absorption spectrum of which the maximum is in the shorter wavelength range of the visible spectrum so that normally a second filter dye is needed to block or absorb green light and even a third one to absorb radiations of longer wavelengths e.g. radiations in the red or even in the infrared region. Another aspect concerning decolouration under the influence of suiphite ions in alkaline aqueous liquids is related to the decreasing reaction velocity with sulphite ions going from mono- to trimethine dyes.
Once a filter dye has been selected, the problem is how to get that filter dye in a coated layer so that all the requirements mentioned previously are met.
One of the possibilities is to make use of solid particle dispersions of water insoluble dyes as has been described in EP-A's 0 384 633; 0 323 729; 0 274 723; 0 276 566 and 0 351 593 and in U.S. Pat. Nos. 4,900,653; 4,904,565; 4,949,654; 4,940,654; 4,948,717; 4,988,611 and 4,803,150.
Another possibility is offered in Research Disclosure 19551 (July 1980) which describes an approach of associating hydrophobic compounds with latex polymer particles.
EP-A 0 401 709 describes the dissolution of hydrophobic dyes into oil droplets being substantially insoluble in water and the preparation of the corresponding oilformer dispersions or loaded polymer latex dispersions.
To prevent dye wandering, the dye is often coated with a mordant to bind the dye in the layer in which it is coated as is, e.g., illustrated in U.S. Pat. No. 2,527,583. As dye mordants polymers are often used.
Another possibility is offered by adsorption of dyes at the surface of very fine light-insensitive silver halide crystals with the expectable disadvantages of the coating of more silver halide crystals and possibly fixation difficulties.
More recently methods of preparing dispersions of photographically useful compounds have been given in EP-Application No. 95202034, filed Jul. 24, 1995. Methods of manufacturing a silver halide photographic material suitable for rapid processing applications has been described in EP-Application No. 95201822, filed Jul. 4, 1995.
Very few dyes satisfy the above requirements especially when rapid processing is concerned. Moreover, apart from the requirement of non-diffusibility and of rapid decolorising or removal by rapid processing that the dyes should meet, they should have high stability in the photographic material, not only under the influence of the ingredients present in the emulsion layers prior to coating, but especially under severe storage conditions of the packed material e.g. under circumstances of high temperatures and high degrees of humidity. The task thus remains to synthesise new dyes and to get them in dispersed form into a photographic material.